Given samples of gold, pyrite and chalcopyrite how
would you distinguish them and what properties
could you use to identity them?Thank you for any
help you can send my way. I appreciate the work
you do.

Question Date: 2018-02-26

Answer 1:

There are many ways to tell the differences
between gold, pyrite and chalcopyrite, up to
different levels of proficiency. I will begin with
the brief introduction of these three compounds.

Gold is golden yellow metal with the
chemical formula Au. It is highly soft,
deductible and good conductor of both heat and
electricity. It is very resistant to air and water
when exposed.

Pyrite, known as fool's gold, is an iron
sulfide with chemical formula FeS2.
It is brittle, has brassy yellow color, which is
very similar to Gold.

Chalcopyrite is a copper iron sulfide with the
chemical formula CuFeS2. It also
has brassy yellow
color, but tarnishes to iridescent blues, greens,
yellows, etc.

I. All of these three have similar
colors, so how
do we discriminate between them? Let me first
mention the ways without much professional
knowledge.

1) Color and shine:
Although all of these three have similar colors,
they do have different properties in terms of
color. Gold is golden color; Pyrite is brass like
shiny color; Chalcopyrite is brassy to golden
yellowish.

Since gold is highly deductible, it
shines in
any angle if you rotate the gold sample. This is
because the light can always get reflected in a
proper surface in gold.

Pyrite will glisten
when its surface catches the light. However,
because of its fewer crystal surfaces available
compared to gold, when you rotate the Pytire
sample, you may only get stronger reflections in
certain angle. That means that Pyrite will look
differently when you rotate the sample.

Chalcopyrite: when it is exposed to air, it
will form other types of oxides, hydroxides and
sulfates, which is why it will tarnish to
iridescent colors like blue, green, etc..

2) Ductility, malleability and hardness:Gold is high malleable and ductile and
the softest
among the three. It has a hardness of 2.5-3 on the
Mohs scale. Pyrite, on the other hand, is very
brittle and the hardest with the hardness
of 6-6.5.

If you use a pestle to grind or use a hammer to
shatter those three samples, you will find the
gold samples to be the softest. On the other hand,
Pyrite will be the hardest and shatters into
smaller pieces. You can think of gold as mud and
Pyrite as glass in a simple way. Chalcopyrite
will behave in between the two.

3) Crystal shape:
Because of the aforementioned properties,
natural gold tends to be irregularly
shapes, while the other two usually have
natural edges and surfaces. Pyrites in natural
form comes as cubes or multifaceted crystals.
For Chalcopyrite, it is predominantly the
disphenoid.

Among the three, Gold will be the heaviest given
the same volume of samples. Or in other words,
given the same amount of samples (same mass), gold
will occupy the least volume. Chalcopyrite will be
on the opposite extreme among these three, while
Pyrite stays in between.

5) Heat treatment:
Gold is very resistant to air and water, even
when heated in air up to 1000C. However, both
Pyrite and Chalcopyrite will react with
O2 in air,
when heated. Those byproducts have different
colors and thus can be used to help
differentiate the three samples.

Pyrite will decompose into Fe S (Iron
Sulfide) and then to
elemental S at 540 Centigrades.
Chalcopyrite: it will oxidizes even without
heating, forming oxides, hydroxides and
sulfates.

6) Chemical response:Gold does not react with water and a lot of other
corrosive acids and bases. But this is not true
for both Pyrite and Chalcopyrite. For instance,
Gold is insoluble in nitric acid
(HNO3), but the
other two will dissolve.

II. On the professional level, there are
many ways
or tools to tell the exact differences between
those three. Such as the conductivity, reflective,
magnetic properties, chemical properties, etc.. To
briefly name a few in terms of techniques or
properties:

1) Conductivity:
Gold is a good conductor of electricity while the
other two are semi-metals.

2) X-ray diffraction
Since the three samples have different atomic
arrangements, which results in different
diffraction or reflection properties, then one common
tool is the X-ray diffraction techniques.The
three samples will give different X-ray patterns
and can be easily discriminated by such a tool.

3) Energy dispersive X-ray spectroscopy
(EDS/EDX)
Energy dispersive X-ray spectroscopy is the tool
to detect the existence of chemical elements in a
compound. Since the chemical formula for those
three samples are quite different, they can be
easily differentiated. In these three samples,
element Au exists only in gold, while element Cu
only exists in Chalcopyrite.